I'm sure it's 100 dollars a year or more. I've returned a lot more than 20 dollars without a receipt and they've never said a word.
I would guess they have a fairly complicated algorithm to decide who is stealing and who isn't. And they might skip 100 and wait to see if you surpass 300 or 500 in 3 years. Just guessing at the numbers but people who shoplift for a "living" must try to do a lot more that.
I don't know if there is a way to show them the receipt after it's no longer useful for getting cash back. I would think there would be, because of course they are interested in distinguishing thieves from others.
Projects drag on. Lots of things keep them from going forward, and I think the HD people know that.